1. Field of the Invention
The present invention is related to a compound diffusion plate structure, backlight module, and liquid crystal display; in particular, to a compound diffusion plate structure capable of increasing brightness and range of angle of view, as well as a backlight module and a liquid crystal display using the compound diffusion plate structure.
2. Description of Related Art
Since the demand on the size of backlight module keeps increasing, conventional small to middle size backlight modules of side-light type have been gradually substituted by direct-light type backlight modules. In a backlight module of side-light type, light enters on one side of light guiding plate, and repeatedly reflects and diffuses inside the light guiding plate based on the total reflection principle in order to enhance the light range and uniformity; afterward, the total reflection is broken by an optically designed micro structure to uniformly emit light out of the light guiding plate, light is then diffused through a diffusion film, and concentrated via an enhancing film to a positive angle of view, then after reflection, providing the required light for the light crystal panel, which does not generate light in itself for image display operations. Since the lamps used in the side-light module are hidden on the side of the light guiding plate and covered by masks, human eye will not directly see these lamps, thus uniformity thereof can be conveniently controlled by the design of the light guiding plate; wherein, the lamps are restricted to the thickness of the light guiding plate, so brightness is usually insufficient, thus enhancing film is needed to increase brightness. However, in general cases, due to the shrinkage of angle of view caused by using the concentrating plate, the final angle of view may not be suitable for multiple viewers to see at the same time. In terms of the direct-light type backlight module applied in current LCD-TV's, as the required size becomes larger, the requirements on injection smoothness in large-sized light guiding plate and tonnage of the injection machines make it more and more difficult to successfully achieve the necessary manufacturing processes for direct-light type backlight module, and the overall weight of the product becomes much heavier; therefore, it is common to see side-light light guiding plate with insufficient brightness used in large-size televisions because the manufacturing process difficulty and cost of direct-light type backlight module is much higher; furthermore, the larger the surface area is, the greater the demand for brightness or luminosity becomes, which means the number of lamps needed increases accordingly, and because the side-light light guiding plate with the side-light type space limitation problem can not accommodate more lamps therein, thus it is incapable of meeting the requirements on large surface and high brightness in present television market. It should be noted that large size liquid crystal televisions no longer use the backlight module of side-light type which requires light guiding plate, but employ direct-light type backlight module of multiple lamps to fulfill the demand for televisions with high brightness and wide angle of view features.
The backlight module, in essence, provides the liquid crystal panel with a light source of uniformity and high brightness, whose basic principle is that it converts a common, similar point-wise or linear light source into a planar light source product with high luminosity and uniform brightness through a terse and effective optical mechanism. The optical design of a general side-light type structure uses a linear light source from a Cold Cathode Fluorescent Lamp in which the emitted light passes through a reflection mask and enters into the light guiding plate, converted from linear light source distribution into a uniform planar light source, then made to subject to the light equalization performed by a diffusion plate and light concentration performed by a prism sheet, in order to enhance the brightness and uniformity in the light source.
In general, light passes through a diffusion plate after emitting from a light source, with poorer orientation in the light emitted from the diffusion plate; therefore, it is necessary to use a prism sheet to correct the direction of light which applies the geometric structure in its design to allow light to refract and totally reflect to achieve the objectives of light concentration and frontal brightness enhancement. However, the effect of brightness increase in the backlight achieved by the above method is merely based on the light convergence from a wider angle of view to a narrower one; consequently the angle of view shrinks, causing a problem of reduced angle range of view when watching the display, so that the user can only view with a limited angle range of view or otherwise it is impossible to clearly see the images shown on the screen of the display, resulting in inconvenience for users to a certain extent. As such, currently there also provides a novel Dual Brightness Enhancement Film (DBEF), which applies the principle of multi-film interference, to achieve the design of brightness enhancement on total angle of view without compressing angle of view. For the liquid crystal television using the direct-light type backlight module, since the plurality of lamps in the backlight module are directly places in the lamp box of the backlight module, light emitted from the lamp will be directly seen by human eyes; so, suppose simply placing the lamp into the box without the design of light equalization, it can not be used as a backlight source since this may cause poor uniformity in the backlight module and brightness variation may occur, resulting in so-called un-even lamp image and lamp phenomenon (referred as MURA problem in Japan). The condition occurring in the lamp source of the direct-light type backlight module is rather analogous to when people directly look at a fluorescent light: the place where people see the fluorescent light tube is most bright, but the brightness in the dark zone between light tubes decrease dramatically. Therefore, the direct-light type backlight module needs a diffusion plate and multiple diffusion films, or even micro structure diffusion films, to mask or diffuse lamp images so as to achieve the purpose of uniform backlight source. Only by this uniform backlight module can the uneven effect in the liquid crystal image display be avoided. The compound diffusion plate according to the present invention can reduce or eliminate the MURA problem, thus providing required uniform illumination for future use in various backlight modules, advertisement light boxes, lighting devices, road lamps, and etc.
Accordingly, having noticed the above-discussed improvable defects and utilized solid long-term relevant experiences in practical operations, together with prudent and insightful observations and researches, in conjunction with, theoretical applications as well, the inventors of the application herein propose the present invention to offer reasonable design for being able to effectively amend the aforementioned defects.